THE CLIMATE KELPIE BLOG: Where have our winters gone?

In recent years, many regions in Southern
Australia that traditionally rely on winter rainfall have experienced a decline
in wet-season totals.

Heatwaves have also become more frequent,
and not just in summer.

While big picture climate drivers, such as
the El Niño Southern
Oscillation, play an important role in individual seasons, a persistent trend
in the behaviour of the subtropical ridge is behind these warmer and drier
conditions.

Figure 1. The subtropical ridge plays an important role in the occurrence of heat waves over southern Australia. For instance, temperatures in south-eastern Australia were substantially above average in January 2019. Source: Bureau of Meteorology.

High pressure system

The subtropical ridge is a belt of
high-pressure systems that circles the midlatitudes of the southern hemisphere and
ranges between about 20 and 40 degrees south. It is created as warm tropical
air continually rises, moves south, then cools and falls.

It is part of the global circulation of air
and is a broadscale climate system feature that has an influence on the weather
in Australia throughout the year.

Bureau of Meteorology’s Senior Research
Scientist Dr Andrew Marshall says that the subtropical ridge has a dominant
influence on Australian climate.

“The belt of high pressure is associated
with clear skies for the parts of the country that sit beneath it.”

“During summer, it tends to sit over the
southern half of the country bringing the warm and dry conditions that we
typically associate with the southern summer?”

“As we move into autumn and winter the
ridge migrates north allowing cold fronts and low-pressure systems to penetrate
the southern states resulting in cool conditions and rainfall.”

Inland Australia tends to be affected by
the subtropical ridge all year round which is why it is the driest part of the
continent.

The ridge is essentially a series of
high-pressure systems that move from west to east and suppress or weaken cold
fronts. Any fronts that do manage to break through tend to be relatively weak.

Figure 2. The subtropical ridge is a band of high pressure over Australia that usually moves north in the winter. Source: Bureau of Meteorology.

Winter rain goes AWOL

“However, in the last three winters (2017,
2018 and 2019) the subtropical ridge has been unusually strong,” said Dr
Marshall.

“It has remained further south than usual,
meaning that southern Australia has experienced prolonged high pressure that
blocks cold fronts and winter westerlies. This leads to clear and dry
conditions with warm days and frosty mornings.”

Climate drivers such as the Southern
Annular Mode (SAM), Indian Ocean Dipole (IOD) and El Niño – Southern
Oscillation (ENSO) have all been shown to influence the strength and position
of the subtropical ridge.

“In 2017 SAM was in its positive phase, in
2018 a higher than normal number of high-pressure systems formed over the
Tasman Sea, and in 2019 the IOD was in its positive phase,” Dr Marshall said.

Weaker westerly winds can also lead to an
increase of easterly onshore winds that transport moist air from the Tasman Sea.
This occurred in June 2017 when the coastline from Sydney to Brisbane received
above-average rainfall.

More heatwaves

The subtropical ridge also plays an
important role in the occurrence of heat waves over southern Australia,
including warm spells during the winter.

When these highs are centered over the
Tasman Sea, warm air is transported southwards to south-eastern Australia,
bringing unseasonably warm conditions that can last for a couple of weeks. For
example, persistent Tasman Sea highs contributed to warmer than average
conditions over much of southern Australia during November 2017, July 2018 and
January 2019.

Figure 3. Persistent high pressure over the Tasman Sea, associated with the subtropical ridge, contributed to widespread heatwaves for southern Australia, particularly over the southeast, in January 2019. Source: Bureau of Meteorology.

A worrying trend

Research also shows a role for climate
change in the strengthening of the subtropical ridge and the related 10 to 20
per cent decrease in rainfall across both southeast and southwest Australia
over recent decades.

While the natural cyclic variability of La
Niña and negative IOD can still bring very wet years, a background trend
towards drier winters is evident.